The present invention relates to devices and methods for delivering a substance or product, including medicinal products, in selected doses or amounts. The product can be administered directly during delivery, for example by injection. The device and method can also, however, be used in conjunction with an administering means, for example a pressure injector or injection means, or an atomising means such as that associated with an inhaler. When delivering product in the latter applications, the desired product dosage is set and delivered into the subsequent, actual administering means.
Known injection apparatus, one of which is, for example, described in WO 99/03522, typically comprise a piston in a reservoir filled with product, wherein the piston is moved in order to deliver the product from the reservoir. The delivery movement of the piston is performed by means of a piston rod. For selecting the product dosage to be delivered and for delivery itself, the known apparatus further comprise a dosing and activating means which is coupled to the piston rod. The known dosing and activating means comprise a dosing member which is in direct engagement with the piston rod. By selecting the dosage, the piston rod is axially advanced a selected distance, relative to the dosing member, towards the piston. The dosing and activating means is then advanced with the dosing member and the piston rod. During this delivery movement, the piston rod presses against the piston, which therefore advances in the reservoir towards a reservoir outlet, thus delivering the selected product dosage. The procedure of selecting the dosage and delivering the product can be repeated until the length of the piston rod available for selecting the dosage is used. In the device of WO 99/03522, the piston rod is a threaded rod and the dosing member a threaded nut which are in threaded engagement with each other. If the piston rod is to be used again for subsequent administering of product, after the reservoir has been filled or exchanged, then it has to be rotated back, relative to the dosing member, along its threaded length used for selecting the dosage, which is awkward.
In another known design of product delivery devices, the piston rod is formed as a toothed rod and the dosing member as an elastic catch which engages with the serrated teeth of the piston rod. In devices of this type, resetting the piston rod is generally not possible.